Hybrid power plant for improved efficiency and dynamic performance

ABSTRACT

A hybrid power plant is characterized by a substantially constant load on generators regardless of momentary swings in power load. Short changes in power load are accommodated by DC components such as capacitors, batteries, resistors, or a combination thereof. Resistors are used to consume power when loads in the power plant are generating excess power. Capacitors are used to store and deliver power when the loads in the power plant demand additional power. Reducing rapid changes in power load as seen by the generators allows the generators to operate at higher efficiencies and with reduced emissions. Additionally, power plants employing combinations of generators, loads, and energy storage devices have increased dynamic performance.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/544,173, filed Aug. 19, 2019, entitled “Hybrid Power Plant forImproved Efficiency and Dynamic Performance,” which is a continuation ofU.S. patent application Ser. No. 15/234,771, filed Aug. 11, 2016,entitled “Hybrid Power Plant for Improved Efficiency and DynamicPerformance,” now U.S. Pat. No. 10,389,113, which is a continuation ofU.S. patent application Ser. No. 13/734,761, filed Jan. 4, 2013,entitled “Hybrid Power Plant for Improved Efficiency and DynamicPerformance,” now U.S. Pat. No. 9,444,252, which is a continuation ofU.S. patent application Ser. No. 12/816,576, filed Jun. 16, 2010,entitled “Hybrid Power Plant for Improved Efficiency and DynamicPerformance,” now U.S. Pat. No. 8,373,949, the disclosures of which arehereby incorporated by reference in their entireties.

TECHNICAL FIELD

This disclosure generally relates to power transmission networks. Morespecifically, this disclosure relates to operating a DC power systemfrom one or more AC or DC power generators. Even more specifically, thisdisclosure relates to improving efficiency of an AC generators whenconnected to a DC bus by providing a nearly constant load to thegenerators.

BACKGROUND OF THE INVENTION

Power transmissions networks can be made of AC systems, DC systems, or acombination of the two. AC power networks have conventionally been usedthroughout the world. However, DC power networks have certainadvantages. DC power networks are easier to design and implement becausethey introduce no reactance into the power system. Higher efficienciesfrom generators can be achieved in DC systems because only real power istransmitted. Additionally, parallelization of power supplies is simplebecause no synchronization is required when additional supplies or loadsare brought onto the network.

Therefore, in power networks that experience large swings in load on thegenerators and require reliable operation, a combination of DC systemsand AC systems is beneficial. One example of such a power network isfound on drilling platforms or vessels to operate onboard thrusters.Drilling vessels are not anchored in the ocean but are dynamicallycontrolled to maintain a desired position in the ocean. Thrusters arepropeller drives that can have variable rotation speed and azimuthalangle of the blades. They are used to maintain a position withinspecified tolerances of a drilling apparatus. These thrusters areoperated by a power supply onboard the drilling vessel. Any failure ofthe power supply can lead to displacement of the vessel out of thetolerances of the drilling apparatus. In such a case, the drillingapparatus would need to be mechanically decoupled and recoupled afterthe power supply is restored and the position of the drilling vessel iscorrected.

One method of facilitating a reliable power supply is to utilize a DCbus for powering thrusters and other components. Such a powertransmission system is demonstrated in FIG. 1. In such a system, thepower supply is generally made of AC generators coupled to an AC-to-DCconverter, such as AC-to-DC converter 112. The AC-to-DC converter placespower from the AC generators on an intermediate DC bus. Each motor orthruster, as well as other devices utilizing the intermediate DC bus, onboard the drilling vessel is coupled to the intermediate DC bus througha DC-to-AC converter.

FIG. 1 is a block diagram illustrating a conventional DC voltage buscoupling multiple AC voltage generation systems to various loads. Powersystem 100 includes generators 102. The generators 102 are coupled to anAC bus 104 through isolators 106. The isolators 106 allow the generators102 to be removed from the AC bus 104 when they are not used or aremalfunctioning. The AC bus 104 is coupled to a transformer 108 tocondition power for transmission to a line 110. An AC-to-DC converter112 is coupled to the line 110 and converts AC power on the line 110 toDC power for output onto an intermediate DC bus 120. Coupled to the DCbus 120 are DC-to-AC converters 130. The DC-to-AC converters 130 convertDC power on the DC bus 120 to AC power that most devices are designed touse. Coupled to the DC-to-AC converters 130 is a line 132 to which loadsmay be connected. A power dissipating device 134 is coupled to the line132, and the power dissipating device 134 may be, for example, athruster. Additionally, a transformer 135 is coupled to the line 132 tocondition power for a load 136. The load 136 may be, for example, alight bulb.

Another example of the motor 134 may be the draw works onboard adrilling platform. The draw works is a machine that reels out and reelsin the drilling line and conventionally includes a large-diameter steelspool, brakes, and a power source. Operation of the draw works to reelin drilling line may require the full capacity of the ship-boardgenerators. However, there are operations conditions where the drawworks may consume zero power. In reverse operation, the draw works maygenerate power that is placed back on the line 132 while gravity assistsreeling out of the drilling line. The power load changes may occurnearly instantaneously.

Rapid changes in the load on the generator require the generator toincrease power output to generate the power demanded by the load. Dieselgenerators are designed to consume fuel at an optimized rate in a smallrange of the available power output. Diesel fuel costs are the highestexpense incurred by operating a diesel generator over its lifetime.Therefore, an operator desires to keep the generator operating in thepower output range optimized for fuel consumption.

Turning now to FIG. 2, a power output curve for a diesel generator areexamined. FIG. 2 is graph illustrating the operation of a dieselgenerator. A curve 220 represents fuel consumption in kilograms perkilowatt-hour of the diesel generator at various engine loads (poweroutput). A range between 0 and 100 percent of rated output demonstratesa variation in the kg/(kw/hour) ratio, or efficiency of fuel consumptionIn order to operate efficiently a range 230 of power load on the dieselgenerator should be maintained. If the load increases or decreases, theengine fuel consumption and efficiency changes.

In addition to fuel consumption issues, scrubbers on diesel generatorsthat reduce the dangerous exhaust are sensitive to the volume ofexhaust. Rapidly varying engine power changes the rate of flow ofexhaust and chemical components of the exhaust. Because the scrubber isdesigned to operate optimally on a continuous and stable flow ofexhaust, emissions output may not be minimized if the power load variesrapidly.

Further, dynamic performance of diesel generators is limited. That is,diesel generators may not increase power output rapidly enough to matchan increasing power load on the diesel generator. Conventionally,additional diesel generators would be brought online if the rate ofincrease of power load exceeds the rate of increase of diesel generatorpower output. Neither diesel generator is operating efficiently andresults in increased fuel consumption and express capacity when thepower load peaks.

Referring now to FIG. 3, generators and power loads will be examined ina conventional power plant. FIG. 3 is a block diagram illustrating powerdistribution on a conventional power plant 300. The power plant 300includes an AC generator 302 coupled to a switchboard 308 through an ACline 306. The switchboard 308 is coupled to multiple loads. For example,typical shipboard and drilling loads are represented by a powerdissipating device 312 coupled to the switchboard 308 by an AC line 310.Additionally, the switchboard 308 is coupled to an AC-to-DC converter318. The AC-to-DC converter 318 is coupled to an AC line 316 and a DCline 320. Additional loads may be coupled to the DC line 320. Forexample, a light 322 may be coupled to the DC line 320 or a DC-to-ACconverter 324. The DC-to-AC converter 324 couples to additional AC loadssuch as a power dissipating device 326. The power dissipating device 326may be a draw works as described above or a motor. Each of the loads312, 322, 326 produces different power loads on the AC generator 302.The effect on the AC generator 302 will now be examined.

FIGS. 4A to 4E are graphs illustrating power consumption in aconventional power plant such as FIG. 3. A line 402 in FIG. 4A indicatespower consumption at the power dissipating device 312. Shipboard loadssuch as the power dissipating device 312 operate as a constant load overlong periods of time such as hours on the AC generator 302. The line 402is positive indicating consumption of power. A line 404 in FIG. 4Bindicates power consumption at the power dissipating device 326. Drawworks such as the power dissipating device 326 operate as a varyingload, which may change rapidly such as in milliseconds, on the ACgenerator 302. The line 404 varies between positive and negative valuesindicating the load consumes power at some times and produces power atother times. A line 406 in FIG. 4C indicates power consumption at thelight 322. The light 322 operates as a constant load over long periodsof time such as hours on the AC generator 302.

Total power transferred through the AC-to-DC converter 318 isrepresented by adding the line 404 to the line 406 and is shown in aline 408 in FIG. 4D. The line 408 is total power consumption withrespect to time of the DC line 320. Total power delivered by the ACgenerator 302 is shown in a line 410 in FIG. 4E and is a sum of lines408, 402. In the conventional power plant 300 power delivered by the ACgenerator 302 varies in time. This leads to undesirable qualitiesexhibited by the AC generator 302 as indicated above includinginefficient fuel consumption and poor exhaust scrubbing.

Thus, there is a need for a power plant design that produces asubstantially constant load on the AC generators and increases dynamicperformance.

BRIEF SUMMARY OF THE INVENTION

A power plant includes an AC generator, an AC-to-DC converter coupled tothe AC generator and a DC bus, and a switch coupled to the DC bus. Thepower plant further includes an active power compensation system coupledto the switch. The active power compensation system reduces power loadvariations in the power plant. The switch may include a DC-to-DCconverter. The active power compensation system may include powerconsumption devices. The power consumption devices may be resistors. Thepower plant may also include power storage devices. The power storagedevices comprise ultracapacitors. The ultracapacitors may be coupled toone or more microcontrollers. The one or more microcontrollers mayregulate the ultracapacitors. The power storage devices may includebatteries or rotating machines.

A method of reducing variations in a power load on a generator includesrouting power between the generator and a power consuming device duringa time when the power load on the generator is lower than a first level.The power consuming device may include a resistive element. The firstlevel may be based, in part, on a fuel efficiency of the generator.

A method of reducing variations in a power load on a power plant havinga generator includes routing power between the generator and a energystorage device during a time when the power load on the power plant islower than a first level. The energy storage device stores energyprovided by the generator. The energy storage device may include atleast one ultracapacitor. The energy storage device may include at leastone battery. The first level may be based, in part, on a fuel efficiencyof the generator. The method may also include routing power between thegenerator and the power storage device during a time when the power loadon the power plant is higher than a second level. The second level maybe higher than the first level. The energy storage device may deliverpower to the power plant. The second level may be chosen, in part, basedon a fuel efficiency of the generator. The method further includesrouting power between the generator and a power consuming device duringa time when the power load on the power plant is lower than a thirdlevel. The third level may be lower than the first level. The thirdlevel may be chosen based, in part, on a capacity of the energy storagedevice.

A power plant includes means for generating power to meet a power loadof the power plant. The power plant also includes means for reducingvariation in the power load of the power plant. The means for reducingvariation may include means for consuming energy. The variation reducingmeans may include means for storing energy.

The foregoing has outlined rather broadly the features and technicaladvantages of the present disclosure in order that the detaileddescription that follows may be better understood. Additional featuresand advantages will be described hereinafter which form the subject ofthe claims of the disclosure. It should be appreciated by those skilledin the art that the conception and specific embodiments disclosed may bereadily utilized as a basis for modifying or designing other structuresfor carrying out the same purposes of the present disclosure. It shouldalso be realized by those skilled in the art that such equivalentconstructions do not depart from the technology of the disclosure as setforth in the appended claims. The novel features which are believed tobe characteristic of the disclosure, both as to its organization andmethod of operation, together with further objects and advantages willbe better understood from the following description when considered inconnection with the accompanying figures. It is to be expresslyunderstood, however, that each of the figures is provided for thepurpose of illustration and description only and is not intended as adefinition of the limits of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, reference isnow made to the following descriptions taken in conjunction with theaccompanying drawings.

FIG. 1 is a block diagram illustrating a conventional DC voltage buscoupling multiple AC voltage generation systems to various loads.

FIG. 2 is a graph illustrating the operation of a diesel powergenerator.

FIG. 3 is a block diagram illustrating power distribution on aconventional power plant.

FIGS. 4A to 4E are graphs illustrating power consumption in aconventional power plant such as FIG. 3.

FIG. 5 is a block diagram illustrating power distribution on anexemplary power plant with power dissipating devices to consumeregenerated energy according to one embodiment.

FIGS. 6A to 6F are graphs illustrating power consumption in an exemplarypower plant with resistors to consume regenerated energy according toone embodiment.

FIG. 7 is a block diagram illustrating power distribution on anexemplary power plant with active power compensation according to oneembodiment.

FIGS. 8A to 8G are graphs illustrating power consumption in an exemplarypower plant with active power compensation according to one embodiment.

FIGS. 9A to 9G are graphs illustrating power consumption in an exemplarypower plant with active power compensation and a capacity limited energystorage device according to one embodiment.

FIG. 10 is a block diagram illustrating an exemplary active powercompensation system according to one embodiment.

DETAILED DESCRIPTION OF THE INVENTION

Reducing variation of the load on a generator in a power plant may beaccomplished by adding devices that dissipate power during short timeswhen power loads are volatile. In this arrangement, the generator may beable to continue operation at a higher output while the powerdissipating devices remove power generated by some loads. Without thepower dissipating devices to remove energy generated by the loads, thegenerators would reduce power output and allow other loads to absorb theregenerated power.

FIG. 5 is a block diagram illustrating power distribution on anexemplary power plant with power dissipating devices to consumeregenerated energy according to one embodiment. A hybrid power plant 500includes an AC generator 502 coupled to a switchboard 508 through an ACline 506. The switchboard 508 is coupled to the AC line 506 and an ACline 510. A power dissipating device 512 is coupled to the AC line 510.The power dissipating device 512 may represent, for example, shipboardloads. The switchboard 508 is also coupled to an AC-to-DC converter 518through an AC line 516. The AC-to-DC converter 518 provides power to aDC line 520. A light 522 couples to the DC line 520. Additionally, aDC-to-AC converter 524 is coupled to a power dissipating device 526 andthe DC line 520. The power dissipating device 526 may be a draw works asdescribed above. Additionally, a DC-to-DC converter 532 couples a powerdissipating device 534 to the DC line 520. The power dissipating device534 may be any device capable of consuming energy. For example, thepower dissipating device 534 may be a resistor, variable resistor, waterbrake, or a combination of the aforementioned devices. The power demandon the AC generator 502 from the loads 512, 522, 526, 534 will now beexamined.

Referring to FIG. 6 the loads at various locations on the hybrid powerplant 500 are examined. FIGS. 6A to 6F are graphs illustrating powerconsumption in an exemplary power plant with resistors to consumeregenerated energy according to one embodiment. A line 602 in FIG. 6Aindicates power consumption at the power dissipating device 512.Shipboard loads such as the power dissipating device 512 operate as aconstant load over extended periods of time on the power plant. A line606 in FIG. 6C indicates power consumption at the light 522. The light522 operates as a constant load over extended periods of time on thehybrid power plant 500. A line 604 in FIG. 6B indicates powerconsumption at the power dissipating device 526. Draw works such as thepower dissipating device 526 have a power load that varies rapidly withtime in as small as millisecond intervals. In the case of powerdissipating device 526, the power load is positive at some times andnegative at other times. During the positive portion of the line 604 thepower dissipating device 526 consumes power; during the negative portionof the line 604 the power dissipating device 526 delivers power to thepower plant.

During a time when the power dissipating device 526 is delivering powerto the hybrid power plant 500 the AC generator 502 will reduce poweroutput to accommodate the regenerated power. As described above, the ACgenerator 502 loses efficiency when its power output is reduced orchanges rapidly. Therefore, the power dissipating device 534 may beswitched on by the DC-to-DC converter 532 to consume excess power on theDC line 520. This allows the AC generator 502 to continue operating at anearly constant power output. A line 608 in FIG. 6D indicates powerconsumption by the power dissipating device 534. The line 608 ispositive because the power dissipating device 534 is only capable ofconsuming power. The DC-to-DC converter 532 is switched on at times thatit would be advantageous to add additional power consumption to thehybrid power plant 500. According to one embodiment, the line 608represents power consumption substantially equal in magnitude to theline 604 during the period of time that the line 604 is negative.Therefore, the power dissipating device 534 consumes power generated bythe power dissipating device 526. The DC-to-DC converter 532 may beswitched on for a longer time or shorter time depending on the conditionof other loads on the hybrid power plant 500.

Total power transferred through the AC-to-DC converter 518 is indicatedby a line 610 in FIG. 6E. The line 610 is a summation of the lines 604,606, 608. Total power delivered by the AC generator 502 is indicated bya line 612 in FIG. 6F. The line 612 is a summation of the lines 610,602. The line 612 indicates the load on the hybrid power plant 500 isconfined to a more narrow range than that of the line 410 in FIG. 4E inwhich no power dissipating device is implemented. For example, the line612 has a minimum of 1 MW whereas the line 410 has a minimum of 0 MW Theaddition of the power dissipating device 534 and the DC-to-DC converter532 limits power output reduction of the AC generator 502 when one ofthe loads in the hybrid power plant 500 generates power. The mostinefficient operating range of the AC generator 502 is at low poweroutput, therefore, efficiency of the AC generator 502 in the hybridpower plant 500 is improved by not operating the AC generator 502 at lowpower loads.

The power plant may be further adapted to increase efficiency if theenergy generated by loads may, instead of being dissipated, be storedand used at a later time when power demand increases. As a result, anincrease in load on the power plant would result in a discharge of thestored energy allowing the AC generator to continue operating at anearly constant engine power load. A system for storing energy anddelivering energy depending on conditions in the power plant is referredto as an active power compensation system.

FIG. 7 is a block diagram illustrating power distribution on anexemplary power plant with active power compensation according to oneembodiment. A hybrid power plant 700 includes a energy storage device744 coupled to the DC line 520 through a DC-to-DC converter 742. Theenergy storage device 744 may be switched on by the DC-to-DC converter742 when additional power should be delivered to the DC line 520. Theenergy storage device 744 may also be switched on at times when excesspower is delivered to the DC line 520 such that the energy may be storedby the energy storage device 744. The energy storage device 744 may beany energy storing device including, but not limited to, spring tension,fuel cells, flywheels, capacitors, variable capacitor, ultracapacitors,batteries, or a combination of the aforementioned devices. In additionto energy storage device 744, the hybrid power plant 700 may, in oneembodiment, also include the power dissipating device 534 coupled to theDC-to-DC converter 532.

Turning now to FIG. 8, the load on the hybrid power plant 700 at variouslocations will be examined. FIGS. 8A to 8G are graphs illustrating powerconsumption in an exemplary power plant with active power compensationaccording to one embodiment. The lines 602, 604, 606 of FIGS. 8A, 8B,and 8C, respectively, are identical to those in FIG. 6. A line 809 inFIG. 8E indicates power load of the energy storage device 744. The line809 has substantially the same magnitude as the line 604, but ofopposite polarity. The line 809 is a mirror image of the line 604. Theenergy storage device 744 stores energy during periods of excess powergeneration and delivers energy during periods of power generationshortage. As a result, variations in power load on the AC generator 502are reduced. The reduction is a result of the energy storage device 744consuming power during time that the power dissipating device 526 anddelivering that power back to the hybrid power plant 700. A line 808 inFIG. 8D indicates the power load on the power dissipating device 534.Power load at the AC-to-DC converter 518 in the hybrid power plant 700is indicated by a line 810 in FIG. 8F. The line 810 is a summation ofthe lines 808, 809, 606, 604 and is a substantially constant value. Aline 812 in FIG. 8G indicates total power load on the AC generator 502and is a summation of lines 810, 602 and is also a nearly constantvalue.

Thus, the use of the energy storage device 744 reduces the effects of avarying power load on the AC generator 502. The energy storage device744 may adapt to changes in the power load of the power dissipatingdevice 526 and other loads in the hybrid power plant 700. The nearlyconstant power load on the AC generator 502 allows for continuousoperation in the most efficient operating region of the AC generator502. Additionally, the energy storage device 744 increases dynamicperformance of the hybrid power plant 700. The AC generator 502 inresponse to an increasing power load may not be capable of increasingoutput quickly enough to match the increasing power load. The energystorage device 744 may have a shorter response time to the increasingpower load and deliver additional power while the AC generator increasesoutput to match the power load on hybrid power plant 700. According toone embodiment, the improved dynamic performance of the hybrid powerplant 700 having the energy storage device 744 allows the AC generatorto remain at a substantially constant power output.

The power dissipating device 534, in one embodiment, is used to consumepower when power generation by the power dissipating device 526 exceedsa capacity of the energy storage device 744. FIGS. 9A to 9G are graphsillustrating power consumption in an exemplary power plant with activepower compensation and a capacity limited energy storage deviceaccording to one embodiment. The line 909 in FIG. 9E represents power atthe energy storage device 744. According to one embodiment, the energystorage device 744 has an energy capacity of 1 megaJoule. During powerconsumption of line 604, the line 909 is negative indicating the energystorage device 744 is providing power. During power generation of theline 604, the line 909 is positive indicating the energy storage device944 is storing power. As the energy storage device 744 reaches a maximumenergy capacity at time t2, the power dissipating device 534 will engageto absorb regenerated power from the load 526 in order to maintain asubstantially constant load on the AC generator 502. The actual energycapacity of the energy storage device 744 may vary from the embodimentdemonstrated. The line 908 in FIG. 9D illustrates that during theportion of time that the energy storage device 744 is near capacity, thepower dissipating device 534 consumes power. As a result, the summationof the switchboard 508 yields the same power load as in FIG. 8.

FIG. 10 is a block diagram illustrating an exemplary active powercompensation system according to one embodiment. An active powercompensation system 1000 may be employed to store and deliver energy tothe hybrid power plant 700. An input line 1012 is used to connect theactive power compensation system to a power plant. The active powercompensation system 1000 includes several columns 1034 of power storagedevices. Each column 1034 includes energy storage devices 1042. Theenergy storage devices 1042 may be, for example, ultracapacitors,capacitors, batteries, or fly wheels. The energy storage devices 1042are stacked in series to obtain a desired voltage and in columns 1034 toobtain a desired current or optimal energy density. The energy storagedevices 1042 are controlled by microcontrollers 1044 to regulatecharging and discharging activities. For example, the microcontrollers1044 may disconnect defective or damaged power storage devices 1042 fromthe columns 1034.

Examples of hybrid power plants for drilling vessels including shipboardloads have been shown in the above embodiments. However, the powerplants as disclosed may be adapted for use in a number of otherapplications. Additionally, the power plants may include AC or DCgenerators and loads. AC-to-DC, DC-to-AC, and DC-to-DC converters asshown in the figures above may be unidirectional or bidirectional. Oneof ordinary skill in the art would be capable of substitution, e.g., anAC-to-DC for a DC-to-AC converter, depending upon load configuration andcharacteristics (i.e., DC load or AC load) of a particular power plant.

Although the present disclosure and its advantages have been describedin detail, it should be understood that various changes, substitutionsand alterations can be made herein without departing from the spirit andscope of the disclosure as defined by the appended claims. Moreover, thescope of the present application is not intended to be limited to theparticular embodiments of the process, machine, manufacture, compositionof matter, means, methods and steps described in the specification. Asone of ordinary skill in the art will readily appreciate from thepresent invention, disclosure, machines, manufacture, compositions ofmatter, means, methods, or steps, presently existing or later to bedeveloped that perform substantially the same function or achievesubstantially the same result as the corresponding embodiments describedherein may be utilized according to the present disclosure. Accordingly,the appended claims are intended to include within their scope suchprocesses, machines, manufacture, compositions of matter, means,methods, or steps.

What is claimed is:
 1. An apparatus, comprising: an AC power sourcecoupled to an AC bus; a bidirectional AC-to-DC converter coupled to theAC bus and a DC bus; a first AC load coupled to the AC bus; and anactive power compensation system coupled to the DC bus, wherein theactive power compensation system comprises an energy storage devicecoupled to the DC bus through a switch.
 2. The apparatus of claim 1,further comprising a summation block on the AC bus coupled to the ACpower source, the AC load, and the bidirectional AC-to-DC converter,wherein the active power compensation system is configured to maintainan approximately constant load on the AC power source through thesummation block.
 3. The apparatus of claim 1, further comprising: aDC-to-AC converter coupled to the DC bus; and a second AC load coupledto the DC bus through the DC-to-AC converter.
 4. The apparatus of claim3, wherein the DC-to-AC converter is bidirectional to allow powerregeneration of the motor to provide power to the DC bus.
 5. Theapparatus of claim 1, wherein the switch comprises a DC-to-DC converter.6. The apparatus of claim 1, wherein the energy storage device comprisesat least one of an ultracapacitor, a capacitor, a battery, and a flywheel.
 7. A method, comprising: energizing an AC bus from an AC powersource; powering an AC load with the AC bus; transferring energy fromthe AC bus to a DC bus through a bidirectional AC-to-DC converter; andstoring energy in a power storage device from the DC bus by transferringenergy through a switch.
 8. The method of claim 11, further comprisingmaintaining an approximately constant load on the AC power source byrouting power between the AC power source, the AC load, and the powerstorage device through a summation block.
 9. The method of claim 11,further comprising powering a second AC load from the DC bus through aDC-to-AC converter.
 10. The method of claim 13, further comprisingproviding power to the DC bus from the second AC load through theDC-to-AC converter.